Special Relativity 2: Spacetime & Space Travel eBook

Our Universe 2:
Redshift, Expansion,
& Dark Energy

Robert L. Piccioni, Ph.D.

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This book continues our exploration of cosmology, the science of the universe as a whole.

One of the most profound questions in cosmology is: Has our universe existed forever, or did it have a beginning? Philosophers, theologians, and scientists have argued both sides of this debate, for millennia. The first observational evidence came in 1929 when American astronomer Edwin Hubble announced his discovery that the universe was expanding, and therefore had a beginning. This was one of the most important discoveries of the 20th century.

This eBook examines the key steps leading to Hubble’s discovery, including the contributions of Henrietta Leavitt and the understanding of supernovae and redshifts, color changes of starlight that reveal motion. We explain what “expansion” means, what is expanding and what isn’t. We then explore the accelerating expansion of our universe and dark energy.


Type 1a supernovae result from white dwarfs exceeding their mass limit. White dwarfs are collapsed cores of stars that can no longer sustain nuclear fusion. The primary mechanism for white dwarfs growing too massive is by cannibalizing nearby companions. In binary star systems (two stars held together by their mutual gravity), the heavier star will evolve more rapidly and can become a white dwarf while its partner is still in the red giant stage.

Red giants are old stars past the hydrogen fusion stage that have greatly expanded in size, by as much as 100-fold.

Illustrated above is a white dwarf (left) accreting gas and growing at its partner’s expense, a nearby red giant (right).

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Our Universe 2:
Redshift, Expansion
& Dark Energy

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